1. Field of the Invention
This invention relates to a light emitting device that a solid-state device such as an LED (light emitting diode) element is sealed by a sealing material and, particularly, to a light emitting device that can prevent an electrical interference, such as a disconnection in bonding wire and a contact separation, due to a difference in thermal expansion coefficient between a housing for mounting the solid-state element and the sealing material.
2. Description of the Related Art
Japanese patent application laid-open No. 2002-314142 discloses a light emitting device that an LED element is sealed with a transparent resin material.
FIG. 1 is a cross sectional view showing the conventional light emitting device. The light emitting device is composed of a LC (liquid crystal) polymer resin housing 103, a GaN system semiconductor light emitting element 106 mounted on the housing 103, a silicone sealing material 111 covering the GaN system semiconductor light emitting element 106, and leads 101, 102 such as a metal lead frame. The leads 101, 102 are integrated with the LC polymer resin housing 103. The leads 101, 102 are disposed such that its end is close and opposite to each other.
The LC polymer resin housing 103 is composed of a cone-shaped reflection surface 104 and a bottom surface with the leads 101, 102 exposed thereon. The other end of the leads 101, 102 extends to a direction opposite to each other and is drawn out of the LC polymer resin housing 103.
The GaN system semiconductor light emitting element 106 is mounted on the bottom surface of the housing 103. The cone-shaped reflection surface 104 may be an ellipsoidal surface or revolution paraboloidal surface.
The light emitting element 106 is mounted through an adhesive such as Ag paste on the lead 101 at the bottom of the cone-shaped reflection surface 104. The GaN system semiconductor light emitting element 106 is provided with first and second electrodes (not shown) with which the leads 101, 102 are connected through bonding wires 108, 109 such as a gold (Au) wire.
The silicone sealing material 111 filled in the cone-shaped reflection surface 104 contains a phosphor 110. The emission peak wavelength of the light emitting element 106 may be, for example, less than 400 nm, and the phosphor 110 may be excited by primary light of less than 400 nm. Also, the phosphor 110 may be of one type or composed of a red emission phosphor 110A, a green emission phosphor 11B and a blue emission phosphor 110C.
In such a composition, primary light emitted from the GaN system semiconductor light emitting element 106 is externally discharged while being wavelength-converted by the phosphor 110 without being discharged as it is. In other words, the primary light such as ultraviolet light emitted from the light emitting element 106 is wavelength-converted by the phosphor (e.g., the red phosphor 110A, green phosphor 110B and blue phosphor 110C) and discharged as a combined light of secondary lights.
Silicone has a refractive index slightly lower than epoxy resin and therefore it is at a little disadvantage in light extraction from the light emitting element. However, it is less unlikely to be yellowed due to light or heat discharged from the light emitting element, and it can therefore prevent a reduction in light output of the light emitting device caused by the yellowing.
However, the conventional light emitting device has problems as described below.
The silicone sealing material 111 is composed of silicone with a thermal expansion coefficient as large as 200×10−6/° C. (which is greater than that of epoxy resin). On the other hand, when the LC polymer resin housing 103 and the leads 101, 102 are replaced by Al2O3 as a ceramic resin material and a metallic pattern formed thereon, they have a thermal expansion coefficient of 8×10−6/° C. (which is smaller than that of glass epoxy resin). Further, the housing 103 has an opening at the top. As a result, the difference between the housing 103 and silicone sealing material 111 becomes about twenty five times. Therefore, when the light emitting device is treated or operated in high-temperature environments, an upward force will be applied to the silicone sealing material 111 at the bottom corner portion of the cone-shaped reflection surface 104. This may cause an electrical interference such as a disconnection in the bonding wires 108, 109 and a contact separation at the n- and p-electrodes. Furthermore, the sealing property maybe reduced due to the separation between the silicone sealing material 111 and the cone-shaped reflection surface 104 formed on the resin housing 103.